ISSUE 134, AUGUST 2013 2 Imperative: Venus Continued
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Imperative: Venus — Virgil L. Sharpton, Lunar and Planetary Institute Venus and Earth began as twins. Their sizes and densities are nearly identical and they stand out as being considerably more massive than other terrestrial planetary bodies. Formed so close to Earth in the solar nebula, Venus likely has Earth-like proportions of volatiles, refractory elements, and heat-generating radionuclides. Yet the Venus that has been revealed through exploration missions to date is hellishly hot, devoid of oceans, lacking plate tectonics, and bathed in a thick, reactive atmosphere. A less Earth-like environment is hard to imagine. Venus, Earth, and Mars to scale. Which L of our planetary neighbors is most similar to Earth? Hint: It isn’t Mars. PWhy and when did Earth’s and Venus’ evolutionary paths diverge? This fundamental and unresolved question drives the need for vigorous new exploration of Venus. The answer is central to understanding Venus in the context of terrestrial planets and their evolutionary processes. In addition, however, and unlike virtually any other planetary body, Venus could hold important clues to understanding our own planet — how it has maintained a habitable environment for so long and how long it can continue to do so. Precisely because it began so like Earth, yet evolved to be so different, Venus is the planet most likely to cast new light on the conditions that determine whether or not a planet evolves habitable environments. NASA’s Kepler mission and other concurrent efforts to explore beyond our star system are likely to find Earth-sized planets around Sun-sized stars within a few years. The Venus-Earth comparison will be Icritical in assessing the likelihood that “Earth-sized” means “Earth-like” for these discoveries. Exploration of Venus already has shown that planetary mass and distance from the parent star are not sufficient predictors of habitability. Further exploration addressing why and when the evolutionary pathways of Venus and Earth diverged therefore seems essential if we are to accurately gauge the likelihood of distant planets developing and maintaining habitable environments. Has Venus always been uninhabitable? Or is its current environment a product of evolving exogenic or endogenic conditions? Was Venus’ surface ever cool enough to sustain standing water, the coin of the habitability realm? Currently Venus’ atmosphere is 4 to 5 orders of magnitude dryer than Earth’s. Its atmospheric D/H (the ratio between heavy and light hydrogen), however, is ~150 times Earth’s value. This is taken to indicate that a vast amount of water has been lost from Venus and has fueled speculations Bthat Venus may once have held an ancient ocean. Whether or not any of Venus’ water condensed prior to loss, however, depends on outgassing and photolytic loss rates as well as past atmospheric conditions that are poorly constrained. Some theoretical support for a cooler, perhaps habitable ancient Venus comes from the Standard Solar Model, which predicts that the Sun has steadily brightened over its lifetime. This implies that about 4 billion years LUNAR AND PLANETARY INFORMATION BULLETIN • ISSUE 134, AUGUST 2013 2 Imperative: Venus continued . ago — when life was emerging on Earth — the Sun’s luminosity was only 70% of its current value. While Mars and perhaps Earth were frozen wastelands, Venus may have been the only place in the solar system where liquid surface water was in abundant, planet-wide supply. Whether or not Venus was actually cooler early in its history, however, hinges critically on when the L current “super greenhouse” began. Without atmospheric effects even today’s Venus would have a globally averaged temperature of ~50°C, cool enough to retain liquid water on its surface. The additional greenhouse component due to atmospheric water vapor, however, could overwhelm any Atmospheric layers and wind speeds measured by the VIRTIS earlier mitigating effects of reduced instrument onboard ESA’s Venus Express. Credit: R. Hueso solar luminosity. Atmospheric (Universidad del País Vasco). measurements by themselves cannot constrain the sequence of events P needed to reveal whether Venus’ water ever resided on its surface or, for that matter, whether Venus was ever fundamentally different than it is today. The only plausible way to retrace critical steps in Venus’ evolution is to decipher its geological record in detail. Magellan mapped Venus from orbit in the early 1990s, and since then only the European Venus Express mission (since 2006) has lingered. These missions revealed a planetary surface dominated by volcanic landforms and extensive lava plains that have been deformed to varying degrees by minor folding and faulting. The quasi-random distribution of impact craters on Venus and the small number that have been conspicuously modified from the outside by volcanic flows have been used to support a model wherein the present volcanic surface of Venus was emplaced rapidly (in as little time as 10 million years) Isometime between 1100 and 350 Ma. This “catastrophic” interpretation of the globally averaged crater retention age of Venus is intriguing but remains controversial. First of all, crater densities are too low to constrain the ages of specific geological units or landforms on Venus. Furthermore, the interpretation that the vast majority of venusian craters are “pristine” has been disputed by studies indicating that craters with smooth, radar dark floor deposits are 150–400 m shallower than those with radar bright (rough) floors. These observations indicate that dark floor craters may have been partially filled by lavas extruded during formation of the surrounding plains. If this is in fact the case, then up to 80% of the existing craters were formed prior to the surrounding plains, and the mean surface age is considerably younger than previously estimated. This scenario would extend almost indefinitely the time interval needed to emplace the assemblage of units and landforms that Bmake up the current surface of Venus. In point of fact, therefore, current constraints permit a spectrum of scenarios ranging from catastrophic resurfacing of (virtually) the entire planet with little to no subsequent volcanic activity, to more “steady- state” models (where over a longer time span Venus is being resurfaced a small region at a time). Each of these models is defended to a degree far in excess of that justified by current observational constraints. LUNAR AND PLANETARY INFORMATION BULLETIN • ISSUE 134, AUGUST 2013 3 Imperative: Venus continued . This debate — over one of the most fundamental aspects of Venus evolution — is likely to rage until new observations are made that will allow various models to be vigorously tested and either discarded or refined. The resurfacing style of Venus is not our only enigma. While Venus exhibits extensive rift systems, large areas of heavily deformed terrain L (tessera), and even linear mountain belts, there are no morphological or geophysical indicators of a vigorous (i.e., Earth-like) regime of plate tectonics. Instead, the distribution and morphology of Venus’ tectonic features and the correlation between its surface topography and gravity anomalies suggest that Venus currently has a thick, dry lithosphere much like smaller “one-plate” planets. How a Venusian impact craters with radar dark floor deposits such as those on P large planet such as Venus balances the left are typically shallower and older than those with bright floor deposits (right). its internal heat generation with heat loss through this “stagnant lid” configuration is a major conundrum. Earth loses approximately 70% of its heat by recycling relatively cold, dense lithosphere back into the underlying hot mantle during subduction. Conduction across the thick venusian lithosphere would not be adequate to sustain thermal equilibrium between the interior and surface if abundances and distribution of heat-producing I elements are similar to Earth’s. How a solution to this mystery is approached depends on how the average surface age is interpreted. The cornerstone of catastrophists is the episodic-catastrophic resurfacing model wherein internal heat builds to some critical level needed to trigger a global catharsis of lavas, essentially resurfacing the whole planet. The associated cooling of the upper mantle leads to a quasi-stable Overlapping, steep-sided “pancake” domes located period of hundreds of millions of years during southeast of Alpha Regio. Morphology and preliminary which the internal heat again increases and the B cycle begins anew. infrared emissivity suggest these domes contain siliceous materials such as andasite, which on Earth are associated with magmatic water. Magellan image is Another related geodynamic model proposes that a 150 kilometers wide. transition between “thin-lid” tectonics (with plate LUNAR AND PLANETARY INFORMATION BULLETIN • ISSUE 134, AUGUST 2013 4 Imperative: Venus continued . recycling) and a stagnant-lid regime occurred in the recent past (around 300–700 Ma) but does not fully address the issue of long-term internal heat buildup beneath the thickened lithosphere. Whether or not Venus ever developed a regime of plate tectonics is unknown. Improving basic observational constraints holds the only hope of resolving this issue and gaining a more reliable understanding of how internal and external activity are coupled and have evolved. The two end-member resurfacing models (catastrophic and steady state) predict dramatically different styles and rates for the geological activity that